The current default radius used in Google Maps distance calculations
is the WGS-84 equatorial radius, "a", 6378.137 km (6378137 m).
While "a" is ideal along the equator, that is only one extreme: The
other is along a north-south meridian, which approximately equals "[.5*
(a^2+b^2)]^.5", the "elliptical quadratic mean". For WGS-84, this
works out to 6367.454, with the actual meridional value being about
6367.449 km.
If you average the two extremes, you get the "ellipsoidal quadratic
mean", "Qr":
/ equatorial^2 + meridional^2 \^.5
Qr = (-------------------------------),
\ 2 /
/ a^2 + .5*(a^2+b^2) \^.5
= (----------------------),
\ 2 /
/ 3*a^2 + b^2 \^.5
= (---------------);
\ 4 /
So, for the WGS-84 spheroid model, where a = 6378.137 and
b = 6356.752314, Qr ~=~ 6372.7975559, 6372.797 or——given all of the
different "flavors" discussed in the "Talk:Earth" link given in the
link below—— even just rounded to "6372.8"! See:
http://groups.google.com/group/comp.infosystems.gis/msg/5897cf05c924a569
for a fuller explanation why, as well as the Wikipedia link to an even
much more thorough explanation.
~Kaimbridge~
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